How to Answer "Modular Monolith" in System Design Interviews

Plan for 35 to 45 minutes total. Spend 5 minutes on requirements and estimations, 10 minutes on high-level design, 15 minutes on detailed design with one or two deep dives, and 5 to 10 minutes on trade-offs and scaling discussion. Always start with requirements even if you think you know the answer because it shows the interviewer that you approach problems systematically.

The expected level of detail depends on your target level. For mid-level roles, demonstrate that you can design a working system with the right components. For senior roles, dive deeper into specific components and discuss trade-offs. For staff and above, show that you can evaluate multiple approaches, quantify their impact, and choose the best one for the given constraints.

Generally no. System design interviews focus on architecture, not implementation. However, writing pseudocode for a specific algorithm, a database schema, or an API contract can demonstrate depth. Keep it brief and use it to clarify your design, not to show off coding skills. If the interviewer asks for code, keep it at a high level with clear comments.

Focus on first principles. Break the problem into subproblems you do understand: data modeling, caching, load balancing, and failure handling. Use analogies from systems you have built. Be honest about what you do not know and propose how you would investigate. Interviewers value structured thinking and intellectual honesty more than memorized answers.

Common follow-ups include: How would you scale this 10x or 100x? What happens when this component fails? How would you migrate from the old system to the new one without downtime? How would you handle a global deployment? What monitoring and alerting would you set up? Prepare for these by thinking about each component's failure modes and scaling limits during your initial design.